Great advances have been made in semiconductor technology in the last decade due to the discovery of new fabrication techniques. The revolution in semiconductor technology has been most apparent in the silicon semiconductor industry. Most semiconductor devices are made with silicon, because silicon is readily available in large boules, has excellent mechanical properties, can be easily purified, and is relatively well understood as far as semiconductor properties are concerned.
In fabricating silicon semiconductor devices, it would be highly advantageous to have an etching procedure which can be controlled as to etch rate, area to be etched, and geometrical shape to be etched. Such an etching procedure is usually referred to as an anisotropic etching procedure. Such a procedure would be useful for making any type of three-dimensional features in silicon, for example, channels, via holes, mirrors, lenses, and diffraction gratings.
It is known that photoelectrochemical (PEC) etching may be utilized to implement an anisotropic etching procedure. The PEC etching of III-V semiconductor compounds has been described in a variety of publications, including U.S. Pat. No. 4,482,443 issued to Bacon et al. on Nov. 13, 1984; U.S. Pat. No. 4,389,291, issued to P. A. Kohl et al. on Jun. 21, 1983; U.S. Pat. No. 4,399,004, issued to R. R. Buckley et al. on Aug. 16, 1983; U.S. Pat. No. 4,404,072, issued to P. A. Kohl et al. on Sep. 13, 1983; and D. Lubzens, "Photoetching of InP Mesas for Production of MM-Wave Transferred Oscillators," Electronics Letters, 13, page 171 (1977). Generally, in the prior art, the PEC etching of silicon is performed by placing silicon in an aqueous electrolytic solution containing a fluoride and then exposing the silicon to light radiation.
However, in these aqueous systems for PEC etching, the reactions involve complex interactions between direct fluoride complexation and indirect dissolution through oxide intermediates, such as SiO.sub.2, and the associated liberation of hydrogen gas. In this regard, see H. Gerishcer and M. Lubke, Ber. Busenges. Phys. Chem., 91, 394 (1987) and H. Gerischer and M. Lubke, Ber. Busenges. Phys. Chem., 92, 573 (1988). The formation of the oxide intermediates and their relatively slow dissolution rates results in an overall reaction rate that is not directly proportional to light intensity as desired. The formation or involvement of silicon oxides, which may exist in many different stoichiometries referred to herein generally as SiO.sub.x, requires that the rate of etching as well as the spatial resolution of the etch features be dependent on the dissolution rate of the silicon oxides SiO.sub.x. This predicament leads to low etch rates and lateral diffusion of photogenerated holes which reduces the spatial resolution. Moreover, the formation of hydrogen gas at the silicon surface diffracts the incident radiation, thereby disrupting any imposed spatial gradient of light intensity. The complex reactions also produce surface structures that are strongly dependent on the reaction conditions which are present.